1. Field of the Invention
The present invention is directed to a method for magnetic resonance (MR) imaging, wherein multiple MR tomograms at different positions within a body region to be examined are acquired in temporal succession and presented. The invention is also directed to an MR system for the implementation of such a method.
2. Description of the Prior Art
Magnetic resonance tomography is a known technology for acquiring images of the inside of the body of a living examination subject. For implementation of magnetic resonance tomography, a basic field magnet generative a static, relatively homogeneous basic magnetic field. During the acquisition of the MR tomograms, rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field. Radio-frequency pulses for triggering magnetic resonance signals are emitted into the examination subject with radio-frequency transmission antennas. The magnetic resonance signals produced with these radio-frequency pulses are registered by radio-frequency reception antennas. The magnetic resonance images of the examined body region of the examination subject are produced on the basis of these magnetic resonance signals received with the reception antennas. Each picture element in the magnetic resonance image corresponds to an intensity value of a received magnetic resonance signal of a small body volume. By suitable control of the currents in the gradient coils, MR tomograms can be acquired at different positions and at different angular attitudes, i.e. at different angular positions of the acquired slice relative to the z-axis of the magnetic resonance apparatus.
MR angiography, wherein images of the vascular system of the body region to be examined are acquired, represents a sub-field of magnetic resonance tomography. The required image contrast can be enhanced by introducing a contrast agent into the portrayable vessels before or during the MR exposure. This technique is also known by the name of ceMRA (contrast enhanced magnetic resonance angiography). For visual presentation of a larger area of a vascular system to be acquired, a technique referred to as MIP (Maximum Intensity Protection) is often utilized. All acquired MR tomograms of the vascular system are combined to form a three-dimensional dataset. A family of parallel lines is placed through this dataset. That point having the highest signal intensity along each individual line is selected. Due to the fact that blood vessels having high signal intensity are imaged, exactly one picture element that belongs to a vessel is thus selected along each line. This element is then entered into that projection plane at the end of a line that is perpendicular to the parallel lines. A projection image of the vascular system arises in this way. Different vascular projections are calculated by changing the direction of the projection lines. When these projections are successively presented at a monitor, then an observer receives a spatial impression of the vascular system.
One application of MR angiography relates to MR-guided vessel intervention wherein an interventional instrument introduced into a vessel of a body region is tracked by means of simultaneous MR imaging. The instrument itself, for example a guide wire, a stent or a balloon catheter, can be identified or recognized in the MR tomogram via the artifact caused by its magnetic susceptibility. This technique is referred to as “passive tracking”. The instrument itself can be constructed as an “active” resonant circuit, an “active” antenna or with “active” coils, so the identification of the position of the instrument in the slice can ensue with a local signal pick-up or by a signal super-elevation. This technique is referred to as “active tracking”.
In these applications, the operator of the MR system must manually enter the position and angulation of the 2D slice of the MR tomogram to be acquired via a graphic interface that is overlaid on the MIP presentation of a ceMRA. The checking of the correct positioning ensues by implementing the imaging sequence, i.e. by acquiring the MR tomogram at the input position in the selected angular attitude. In the search for the interventional instrument, this procedure must be repeated for every point along the vessel. This procedure is also repeated after the locating of the intervertional instrument when the latter is moved forward in the framework of the minimally invasive intervention, The search for and the tracking of the interventional instrument are extremely time-consuming compared to a competing imaging technique wherein the tracking of the instrument ensues with conventional X-ray fluoroscopy.
A comparable problem arises in instances wherein an object, for example an endoscope or a probe, introduced into a body canal or tract is to be tracked with MR imaging.